I don't have a preference--as with VirginGo, I like to see characters who end up in a different place, emotionally and mentally, than that in which they were at the opening of the piece.
That said, I can't write a "love story" or "romantic story" or story with a "happy" ending to save my life, so there's that.
Want to spend some time wallowing in a Recommended Read? Pick one! Or two! Or seven!
I find that with erotica, I tend to lean towards happy, but in general I want an ending that fits the story. If you drop a downer in the last chapter of an otherwise upbeat story, likely won't click as well as if there was some foreshadowing of the downer. Ditto happy. I don't want a deus ex machina dropped just to make the ending happy. If the story direction suggests a downer ending, give it one.
I prefer happy endings.
Nawty
I prefer realistic endings and a story that makes hanging in there until the end worth it.